The invention relates to a method and device for calibrating the process of a photographic printing apparatus for copying cinematographic film by projection.
Traditionally two different technologies are used to transform motion pictures from digital format to analogue format:                CRT (Cathode Ray Tube)        Laser        
Motion pictures are composed of a number of picture frames shown one at a time to give the impression of movements. When recording the motion pictures, that is printing on film, one can divide the frames in the original motion picture into lines and write the frame on the film line by line.
In general we can say that both prior art technologies transfer frames in the motion picture to a celluloid film line by line as opposed to the inventive technology that prints on the celluloid film a full frame on each exposure.
CRT Technology
The Cathode Ray Tube (CRT) technique utilises a powerful high resolution picture tube to project film-exposing light in a 3-pass process through a rotating RGB (red, green, blue) colour wheel, or 3 separate picture tubes with separate R, G and B colour filters. It uses three tubes for displaying each of the three basic colours, red, green, and blue. The CRT technique writes one frame at a time and each frame is recorded point by point, line-by-line. There are few moving parts in a CRT machine, and hence relatively little maintenance.
This technique is robust and well known. However, it has several deficiencies. CRT exhibits a restricted contrast range, pronounced grain structure, blooming flare, insufficient colour fidelity and it requires long exposure times. The reason for many of these quality deficiencies can be found in the low light output of Cathode Ray Tubes, the light source used in CRT recorders. To compensate this, a variety of methods are used: a higher speed (=grainier) film is utilised, a lower density range is chosen, brighter, wide-band colour filters are employed which result in insufficient colour separation and low speed recording (15-30 seconds per frame).
Laser Technology
Laser is a complex and very expensive technology. High intensity laser light is reflected off spinning mirrors to expose the film. Laser recorders have heavy maintenance and are prone to very high costs. It is considered to give higher quality output than the CRT at the speed of approximately 5 to 10 seconds per frame. Like the CRT, laser writes each frame line-by-line.
The laser-recorder is dependent on a host computer to process image data and transfer it one line at a time to a line buffer in the recorder. The data in the line buffer is then routed through a digital to analogue converter (DAC) to the input of the acousto-optical modulators (AOMs) which modulate the image data onto the laser beams. Before reaching the AOMs, the light of each laser is adjusted to the required intensity by an attenuator. This is supposed to adjust for variations in film stock sensitivity and compensate for variations in laser energy output. Inside the AOM the laser beam passes through a crystal, and the internal grid structure of the crystal deflects the laser beam. By exiting the crystal with a high frequency wave, the crystal's grid structure, and thus the intensity of the laser beam, can be varied pixel by pixel. The three laser beams are then combined and exposed to the film by a rotating penta prism; one rotation exposes one line.
GB A 2005512 shows a process and apparatus for transcribing a colour videodisc onto a cinematographic film. A colour television signal representing a series of images is read on the video-disc, these images are reproduced on a television receiver, and the screen of this television receiver is filmed with a camera. This corresponds to an enhanced embodiment of the above described CRT recorder.
The greatest disadvantage of the prior art is time consumption. The laser units also have a limited lifetime and replacements are expensive. Hence, the actual recording cost will be higher with a laser recorder than with the device according to the invention. Besides, the laser and CRT recorders' shuttles have to move with extreme precision during the exposure, since any variance in velocity would lead to a variation of the distance between individual lines. This in turn will create a streaking artefact in the image.
U.S. Pat. No. 5,369,433 concerns an apparatus and method for recording a video signal on cinematographic film. A video signal is recorded on unexposed cinematographic film by shining light from a light source onto a deformable mirror device (DMD). The elements of the DMD are controlled in response to the video signal. Light is selectively reflected by the DMD onto an image plane containing the film such that the light reaching the film is representative of the video signal.
This publication deals with transferance of video signals to cinematographic film. In video format pictures are represented by two “half” pictures which in combination gives one picture. 50 such “half” pictures are shown per second, which in effect means that there are 25 whole pictures/sec to be transferred. In cinematographic film, 24 pictures are shown per second, and this difference in frequency must be compensated for. This publication shows use of different devices for transferring information contained in 25 pictures/second to cinematographic film. These devices are deinterlacing and integrating systems etc. The presence of such devices leads to an increase in the total costs and introduces possible error sources.
One problem with prior art is that there are variations in the quality of the unexposed film. This variation is mainly batch variations from the manufacturing of the film, and the variations will influence the final result of the film after exposing. Also variations in the development process for the film, e.g. change in the chemical properties/composition of the development chemicals, will influence the quality of the final film.
This problem is traditionally dealt with by sending sections of exposed and developed film to a laboratory for analysis, followed by adjusting the recording process according to the results of the analysis. This is a very time-consuming method, and the calibration is therefore performed more infrequently than desired.
The object of the present invention is to provide a method for calibration and/or compensation for variations in film stock and chemical changes in the development process in transference of digital and/or analogue motion pictures to a photosensitive material, where the calibration can be performed more frequent than with prior art.
The expression “frame by frame” used in this specification emphasises that the transference is done for each single frame in the source material. This “source material” can be a High definition TV (HDTV) picture sequence, a sequence of pictures scanned from a motion picture film/cinematographic film by means of a full frame digital scanner, a digitally created digital film (e.g. created on a computer), or any other digital full frame formate. With “full frame” in this context it is meant a picture comprising all lines (and pixels) in each frame. This is not the case for TV/video pictures, which consists of two “half” pictures which must be transferred and combined to create a complete picture. The solution related to TV pictures needs complicated interpolation and deinterlacing devices for compensating the difference in number of pictures by second, as mentioned above.
In other words one can say that by means of the invention, transference is achieved without dividing the frame into lines. Besides, the complete full frame of all frames are transferred, this makes deinterlacing and interpolation unnecessary.
A device for transferring full frame digital and/or analogue motion pictures comprising a series of frames, to a photosensitive material, comprises a light source, a colour filter, at least one lens for projecting modulated light onto the photosensitive material a shutter device and a transport device, the device further comprising:                a digital head comprising the light source, the colour filter, at least one full frame device for light modulation, the at least one lens, processing electronics and software for processing the input image data, and for transport and shutter device synchronisation,        a user interface, and        a graphical computer interface connected to the digital head, whereinthe transport device is adapted for exposing, section for section, sections of the photosensitive material corresponding to each of the motion picture frames, to the modulated projected light, in such a way that the number of frames per time unit in the original motion picture corresponds to the number of projected frames per time unit.        
In a further embodiment, the device comprises image sensors located adjacent to the photosensitive material for providing signals to the digital head concerning optical properties of the projected image, the digital head using the sensor signals in controlling the shutter device and the light modulation device.
In a further embodiment, the device for light modulation is a Digital Micromirror Device (DMD), a transmissive LCD or a reflective LCD device.
In a further embodiment the graphical computer interface is adapted for receiving the image data from an external computer.
In a further embodiment the image data has a digital and/or analogue format.
In a further embodiment, the device comprises only one device for light modulation, and this light modulation device is adapted for modulating the three primary colours, red, green, blue.
In a further embodiment the device comprises three devices for light modulation, which each is adapted for modulating one of the three primary colours, red, green, blue.
In a further embodiment the colour filter is a colour wheel divided in sections of different colours, preferably the three primary colours red, green, blue.
In a further embodiment the colour filter comprises separate colour filters of different colours.
In a further embodiment the digital motion picture and/or the analogue representation of that motion picture is either of negative or positive format.
A method for transferring full frame digital and/or analogue motion pictures, comprising a series of frames, to a photosensitive material, comprises, for each frame, the following steps:
a) receiving image data through a graphical computer interface to a digital head,
b) processing the image data in the digital head,
c) modulating light from a light source by means of a device for full frame light modulation according to the processed image data,
d) projecting the modulated light onto one section of a photosensitive material,
e) transferring the photosensitive material, by means of a transport device, to expose the next section,
f) repeating steps d) and e) until the desired sections of the photosensitive material have been exposed to the projected modulated light.
A further embodiment of the method further comprises steps for sensing image properties with image sensors located adjacent to the photosensitive material, for providing signals to the digital head concerning optical properties of the projected image, the digital head using the sensor signals in controlling the shutter device and the light modulation device.
A further embodiment of the method further comprises that the image data has a digital and/or analogue format.
A further embodiment of the method further comprises that the processing of the image data includes converting analogue signals to digital format.
A further embodiment of the method further comprises that a full frame of the motion picture is projected onto the photosensitive material in step d).
A further embodiment of the method further comprises that the exposure of the photosensitive material is performed line by line with continuous transferring of the photosensitive material.
A further embodiment of the method further comprises that the number of frames exposed on the photosensitive material per time unit corresponds to the number of frames per time unit of the original digital and/or analogue motion picture film.